1. Field of the Invention
The present invention relates to a vehicle behavior estimating method and system for estimating an overturn parameter representing the readiness of overturn of a vehicle in running the vehicle and to a vehicle behavior controlling method and system for preventing the overturn of the vehicle by using the vehicle behavior estimating method and system as well as to a body slip angle estimating method and a system for estimating a body slip angle in driving the vehicle.
2. Description of Related Art
Conventionally, there has been known a vehicle behavior control system for preventing an overturn (roll) of a vehicle by estimating the possibility of overturn (roll) of the vehicle in running the vehicle or in turning the vehicle for example by an overturn parameter which represents the readiness of the overturn (roll) of the vehicle and by controlling braking force applied to the respective wheels in correspondence to this overturn parameter.
There has been known a vehicle behavior control system using lateral acceleration acting on the vehicle as the overturn parameter described above. The vehicle behavior control system judges that there is much possibility that the vehicle overturns (rolls) (in other words, the roll angle of this vehicle is excessively large) when the lateral acceleration (the overturn parameter) detected by lateral acceleration sensors or the like mounted in the vehicle exceeds a predetermined value, e.g., 1G, and automatically control the braking force to the respective wheels to prevent the overturn (roll) of the vehicle. In concrete, it prevents the overturn (roll) by applying the braking force to the front wheel on the side of the turning outer wheel of the vehicle and by putting the running state of the vehicle in the tendency of under steering.
However, the control whose overturn parameter is the lateral acceleration is meaningless for vehicles, which do not overturn (roll) even if the lateral acceleration exceeds 1G. That is, when the lateral acceleration exceeds 1G (there is a case when that value is low depending on road surface), grip power of the wheel to the road surface is weakened and no more force which rolls (overturns) the vehicle is applied to the vehicle. Normally, the front wheel of the vehicle cannot keep the original turning course and causes side slip in this case and the vehicle running state is put into the tendency of under steering. Accordingly, it is meaningless to make the control using the lateral acceleration as its overturn parameter because it is unable to accurately represent the readiness of the overturn (roll) of the vehicle in the state in which the lateral acceleration exceeds 1G, i.e., the possibility of overturn (roll) of the vehicle, for the vehicles which do not overturn (roll) and cause only the side slip even when the lateral acceleration exceeds 1G.
Then, the present applicant has proposed a vehicle behavior control system using variation of revolution speed of the wheel as the overturn parameter (shown in Japanese Patent Application No. H.11-72568).
This vehicle behavior control system calculates revolution speed of the turning inner wheel when the turning inner wheel is not floating from the road surface as an estimation value from actual revolution speed of a turning outer wheel and actual lateral acceleration acting on the vehicle in the state in which the wheels are gripped fully to the road surface and calculates and uses an absolute value of the difference between this estimation value and the actual revolution speed of the turning inner wheel as an overturn parameter.
Because there is no friction between the turning inner wheel and the road surface in the state in which the turning inner wheel is floating from the road surface at this time, the revolution speed of the turning inner wheel becomes almost constant when a driver makes no accelerator control or brake control or extremely changes when the driver makes the accelerator control or the brake control. After all, the overturn parameter increases in any case.
When the overturn parameter exceeds a threshold value set in advance, i.e., when the vehicle behavior control system detects that the turning inner wheel has floated from the road surface, it judges that the vehicle is likely to overturn (roll) and prevents the overturn (roll) of the vehicle by automatically controlling the braking force to the respective wheels similarly to the conventional vehicle behavior control system described above.
That is, differing from the conventional vehicle behavior control system described above, this vehicle behavior control system prevents the overturn (roll) of the vehicle by applying the braking force to predetermined wheels only when the turning inner wheels float from the road surface, i.e., only when the control for preventing the overturn (roll) is truly required.
However, there has been a case when it is unable to fully prevent the overturn (roll) of the vehicle when the vehicle runs by changing lanes for example because its timing is too late by making the control for preventing the overturn (roll) after when the turning inner wheel has floated as described above.
That is, the driver turns over the steering before and after the lane change when the vehicle is driven while changing the lanes, so that the vehicle which has once rolled in the direction of the opposite side from the lane changing direction (or the steering direction) (indicated as a roll angle .phi. and a roll rate .phi.' in the vehicle in FIG. 9) rolls in the opposite direction thereof, thus causing a rock-back phenomenon that the direction of centrifugal force F acting on the vehicle is also reversed as shown in FIG. 9.
The amplitude (scale) of this rock-back becomes specifically large when the timing of rock-back coincides with the timing of steering made by the driver. Then, when the timing of rock-back coincides with the timing of steering as such, a vehicle that does not overturn (roll) even when the lateral acceleration becomes 1G during normal turn is liable to overturn (roll) by the equal lateral acceleration of 1G.
Further, although this rock-back differs depending on a type of vehicle and on the degree of attenuation, it occurs as oscillation of about 0.5 to 2 Hz, so that there is a case when the timing is too late and when it is unable to fully prevent the overturn (roll) of the vehicle by making the control for preventing the overturn (roll) after when the turning inner wheel (the wheel on the side of the steering direction in particular in this case) has floated. That is, there is a case when its timing is too late by changing the value of the overturn parameter for the first time when the turning inner wheel (wheel on the side of the steering direction) has floated.
Further, there has been known a body slip angle control as a technology for enhancing control stability in driving a vehicle or in turning the vehicle in particular.
This body slip angle control is a technology for reducing the body slip angle by estimating the body slip angle which is an angle formed between the direction of longitudinal axis of the vehicle body and the actual vehicle advancing direction and by appropriately increasing braking force (wheel cylinder pressure) applied to the front wheel on the side of the turning outer wheel in correspondence to the estimated body slip angle.
It is required to estimate the body slip angle at high precision in the body slip angle control. Hitherto, the body slip angle .beta. has been estimated by calculating an estimation value .DELTA..beta.p of body slip angular velocity .DELTA..beta. by using the following expression based on a yaw rate .DELTA..theta. detected by a yaw rate sensor and the like mounted in the vehicle, lateral acceleration Gy detected by a lateral acceleration sensor and the like and body speed detected by a wheel speed sensor and the like and by integrating it: EQU .DELTA..beta.p=.DELTA..theta.-Gy/Vb (1A)
However, there has been a problem in calculating the estimation value .DELTA..beta.p of the body slip angular velocity .DELTA..beta. based on the above expression (1A) that the estimation value .DELTA..beta.p differs considerably from a true value .DELTA..beta. when the vehicle spins or when a roll angle of the vehicle becomes large in driving the vehicle and that the body slip angle .beta. cannot be estimated accurately as a result.
The reason why the above-mentioned problem occurs will be explained by using FIGS. 19A and 19B.
At first, when the vehicle which is advancing in the direction of a yaw angle .phi. with respect to its running path before turning (path along the x-axis direction) spins and the body slip angle becomes .beta. as shown in FIG. 19A (or when the vehicle which is turning with yaw rate .DELTA..phi. around a circle of radius R shown by a dotted chain line in FIG. 19A as a turning orbit spins and the body slip angle becomes .beta.), the yaw rate sensor mounted in the vehicle detects a value represented by the following expression (2A) as yaw rate .DELTA..theta. based on a yaw rate .DELTA..phi. with respect to the yaw angle .phi. and body slip angular velocity .DELTA..beta. with respect to the body slip angle .beta.: EQU .DELTA..theta.=.DELTA..phi.+.DELTA..beta. (2A)
It is noted that .DELTA..phi. is a yaw rate which is detected by the yaw rate sensor when the vehicle turns along the circle of the radius R as described above without spinning and is described also as an actual yaw rate in the following explanation.
Then, when the vehicle spins as described above (or when the absolute value of the body slip angle .beta. is fully large as compared to zero), the lateral acceleration Gy detected by the lateral acceleration sensor mounted in the vehicle differs from the lateral acceleration Gy' (true value) which actually acts on the vehicle.
That is, because the lateral acceleration sensor detects the component in the direction orthogonal to the direction of the longitudinal axis of the body (the direction described as "Body Direction" in FIG. 19A) in the lateral acceleration Gy' which actually acts on the vehicle, i.e., the component in the direction inclined by the body slip angle .beta. with respect to the direction of the lateral acceleration Gy' which actually acts on the vehicle, as the lateral acceleration Gy, it is unable to detect the lateral acceleration accurately in the state when the lateral acceleration does not act in the direction orthogonal to the body longitudinal direction as the vehicle spins as described above.
Further, the lateral acceleration Gy detected by the lateral acceleration sensor also differs from lateral acceleration Gy' (true value) which actually acts on the vehicle when the roll angle .phi. becomes large as shown in FIG. 19B for example.
That is, the lateral acceleration sensor detects the component in the direction orthogonal to the body vertical axis (see FIG. 19B) in the lateral acceleration Gy' which actually acts on the vehicle, i.e., the component which faces down by the roll angle .phi. with respect to the direction of the lateral acceleration Gy' which actually acts on the vehicle, so that it is unable to detect the lateral acceleration accurately in the state when the roll angle .phi. becomes large as described above and the lateral acceleration does not act in the direction orthogonal to the body vertical axis.
Accordingly, it has been unable to estimate the body slip angle .beta. at high precision as a result by calculating the estimation value .DELTA..beta.p of the body slip angular velocity .DELTA..beta. based on the above-mentioned expression (1A) because the lateral acceleration cannot be detected accurately when the vehicle spins or the roll angle .phi. of the vehicle becomes large and the estimation value .DELTA..beta.p differs considerably from the true value .DELTA..beta..